Air is traditionally separated by fractional distillation at cryogenic temperatures utilizing either a single or double distillation column. Typically, both nitrogen and oxygen products are produced. In addition, by taking an argon-enriched vapor stream from the single or double distillation column and subjecting it to distillation in a further column, an argon product may also be produced. When, for example, both oxygen and nitrogen are required as products in the gaseous state, the incoming air is typically compressed to a pressure in the range of 6-7 atmospheres absolute in a plant using a double distillation column. Since there is a large demand for both liquid nitrogen and liquid oxygen, there is a need for plants that produce both of these products in the liquid state. Liquid oxygen that is collected at the bottom of the distillation column may be taken therefrom as product. The demand for liquid nitrogen may be met taking a gaseous nitrogen product and liquefying at least a portion of it. Modern nitrogen liquefiers typically include compressors that raise the pressure of the nitrogen to 40 atmospheres or more. Alternatively, the liquefaction plant may be integrated with the air separation plant such that the liquid nitrogen product can be taken in substantial quantities directly from the single or double distillation column.
In plants that produce essentially gaseous oxygen and nitrogen products, there have been proposed methods of providing increased argon production which involve the use of an additional liquid-vapor contact column to remix gaseous nitrogen and liquid oxygen and provide additional liquid nitrogen reflux to the column from which the argon-enriched vapor stream is taken for further separation. Such methods are described in European patent application No. 136 926A and international (PCT) patent application No. W087/00609. In addition, UK patent application No. 2 174 916A and European patent application No. 259 070A disclose the use of an additional liquid-vapor contact column to remix gaseous nitrogen and liquid oxygen when argon is required as the sole or primary product of air separation.
There have been no proposals in the art to use the principle of remixing liquid oxygen with gaseous nitrogen to give extra liquid nitrogen reflux in air separation plants that produce substantial quantities of liquid nitrogen product. The invention relates to process, apparatus and plant which employ remixing of liquid oxygen and nitrogen vapor and also produce a liquid nitrogen product.